# std::random_device::entropy

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< cpp‎ | numeric‎ | random‎ | random device

double entropy() const noexcept; |  | (since C++11)

Obtains an estimate of the random number device entropy, which is a floating-point value between 0 and log2(max()+1) (which is equal to std::numeric_limits<unsigned int>::digits). If the device has n states whose individual probabilities are P0,...,Pn-1, the device entropy S is defined as

S = −∑n-1
i=0 Pilog(Pi)

A deterministic random number generator (e.g. a pseudo-random engine) has entropy zero.

### Return value

The value of the device entropy, or zero if not applicable.

### Notes

This function is not fully implemented in some standard libraries. For example, LLVM libc++ prior to version 12 always returns zero even though the device is non-deterministic. In comparison, Microsoft Visual C++ implementation always returns 32, and boost.random returns 10.

The entropy of the Linux kernel device `/dev/urandom` may be obtained using `ioctl RNDGETENTCNT` — that is what `std::random_device::entropy()` in GNU libstdc++ uses as of version 8.1.

### Example

Example output on one of the implementations

Run this code

    #include <iostream>
    #include <random>

    int main()
    {
        std::random_device rd;
        std::cout << rd.entropy() << '\n';
    }

Possible output:

    32

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